The present invention generally relates to a radio receiver for receiving FM stereophonic broadcast signals, and more particularly, to an FM stereophonic receiver which is capable of improving apparent separation between channels by controlling output signal level of a stereo multiplex circuit.
Commonly, for receiving FM stereophonic broadcast signals, it is normally so arranged to subject the broadcast signals to FM detection through employment of an FM detection circuit, and to separate the detection output signal into left and right stereo signals (L) and (R) by using a stereo multiplex circuit for applying said stereo signals to left and right speakers after amplification thereof by an audio frequency amplifier circuit.
In connection with the above, there have also been conventionally proposed a three-channel system which employs, in addition to the left and right speakers, a center speaker to which the stereo sum signal (L+R) contained in the FM detection output signal or stereo sum signal (L+R) prepared through addition of the separated left and right stereo signals is applied, and also, a four-channel system which employs, together with said center speaker, a rear speaker to which the stereo difference signal (L-R) contained in the FM detection output signal or stereo difference signal prepared through subtraction of the separated left and right stereo signals is applied. Thus, by effecting expansion of channels as described above, it becomes possible to improve positions and expansion of sounds for better stereophonic effect.
However, by the mere expansion of channels as described above, it was not yet sufficient to achieve optimum results, and a system capable of improving the stereophonic effect still further has been required for the purpose.